Basic immunology

Introduction:

Immunity is defined as resistance to disease, specifically infectious disease. The immune system is the collection of cells, tissues and molecules that functions to defend us against infectious microbes. Main function of immune system is to prevent or limit infections. This defence function of the immune system is essential for our ability to survive in an environment that has potentially deadly microbes. But, immune responses can also be damaging. Many autoimmune diseases are caused by uncontrolled or excessive immune responses against self antigens (e.g. rheumatic fever, asthma, glomerulonephritis, many dermatological disorders and multiple sclerosis etc.)

Immunity can be classified as:

  • Innate (with which individual is born)   
  • Acquired (which person acquires)

Immunity has mainly two Components:

  • Cell mediated immunity.
  • Antibody mediated immunity.

Two other major components are:

  • Complement system.
  • Phagocytes.

Before we study the Cell mediated immunity (CMI) and Antibody mediated immunity (AMI), here is a brief description about lymphoid system,

Lymphoid system:

The body uses the lymphoid system to enable lymphocytes to encounter antigens and it is here that adaptive immune responses are initiated. The organs involved in specific as well as non-specific immunity are classified as primary (central) lymphoid organs and secondary (peripheral) lymphoid organs. The blood and lymphatic vessels that carry lymphocytes to and from the other structures can also be considered lymphoid organs.

Primary lymphoid organs:

These are sites of antigen-independent lymphocyte proliferation. Here B and T lymphocytes differentiate from lymphoid stem cells into mature albeit naïve effector cells. These include the following:

  • Bone marrow.
  • Thymus.

Secondary or peripheral lymphoid organs:

Secondary lymphoid organs provide an environment that enable lymphocytes to interact with each other, with accessory cells, and with antigens, resulting in the initiation of antigen-specific primary immune responses.

  • Lymph node.
  • Spleen.
  • Peyer’s patches.
  • Tonsils.
  • Adenoids.
  • Appendix.
  • Mucosa-associated lymphoid tissue (MALT).

Diagrammatic representation of human lymphatic system

Lymphatic system

Here is a brief description about these lymphoid organs:

1) Bone marrow:

Bone marrow is the primary source of all the cells of the immune system. During foetal development hematopoiesis occurs initially in yolk sac and para-aortic mesenchyme and later in the liver and spleen. Then it gradually shifts to bone marrow.  The bone marrow is the site of origin of all T- and B- cells, mononuclear phagocytes, platelets, erythrocytes, and other leukocytes in the adult. The bone marrow is divided into wedge-shaped hematopoietic compartments filled with proliferating and differentiating blood cells in connective tissue matrices bordered by venous sinuses.

2) Thymus:

This lymphoepithelial organ develops from ectoderm derived from the third branchial cleft and endoderm of the third branchial pouch including mesenchynmal components derived from cells of the cephalic neutral crest which all migrate from the neck to the anterior mediastinum. It reaches its greatest size just prior to birth, then atrophies with age. Major function of thymus is the production of T-cells or thymocytes. Cells from the bone marrow migrate to the thymus as precursors and develop into mature peripheral T cells. The majority of T cell production occurs before puberty. After puberty, the thymus shrinks and the production of new T cells in the adult thymus drops away. Children with no development of thymus suffer from Di George syndrome that is characterized by deficiency in T cell development but normal numbers of B cells.

3) Lymph nodes:

There are numerous lymph nodes disseminated all over the body. They measure 1–25 mm in diameter and play a very important and dynamic role in the initial or inductive states of the immune response.

Anatomical structure of lymph node:

The lymph nodes are circumscribed by a connective tissue capsule. They receive afferent lymphatics draining peripheral interstitial spaces and also receive blood from the systemic circulation through the hilar arteriole. Lymph node has two main regions: the cortex and the medulla. The cortex and the deep cortex (also known as paracortical area) are densely populated by lymphocytes, in constant traffic between the lymphatic and systemic circulation. In the cortex, at low magnification, one can distinguish roughly spherical areas containing densely packed lymphocytes, termed follicles or nodules.

Structure of lymph node

Structure of lymph node

It is important to note that T and B lymphocytes occupy different areas in the cortex. B lymphocytes predominate in the follicles (hence, the follicles are designated as T-independent area), which also contain macrophages, follicular dendritic cells, and some T lymphocytes.

4) Spleen:

Weighing about 150 grams, spleen is the largest single lymphoid organ in the body. It is situated in the left upper quadrant of the abdomen. It is covered by dense fibrous capsule with muscular trabeculae extending inward to subdivide the spleen into lobules. Its major functions are filtration of blood and it is the major organ in which antibodies are synthesized and released into circulation. Patients with splenectomy (removal of spleen) are highly susceptible to infections with capsulated bacteria such as pneumococci and meningococci.

5) Mucosa associated lymphoid tissue (MALT):

“MALT” refers to a diffuse collection of lymphoid tissues that line the respiratory, alimentary, and genitourinary tracts.  MALT produces the immune responses against pathogens that invade the mucosa that line these tracts.  Like the spleen and lymph nodes, the MALT contains B cell follicles and distinct T cell-rich regions.  In the intestine we find Peyer’s Patches and the tonsils are similar structures in the upper alimentary tract.  Although its organization is similar to the spleen and lymph node, the mucosal immune system is different in several ways:

(1) unlike the spleen and lymph node, MALT tissue is not surrounded by a fibrous capsule,

(2) IgA is the predominant class of immunoglobulin produced in MALT

(3) It is not filtering Ag delivered by vessels like the blood vessels or lymphatic vessels.

 

Know more……………..

Lymphatic circulation:

Blood circulating through the body leaks into tissues through the thin walls of the capillaries. This portion of blood plasma that escapes is called as interstitial fluid. Lymphatic capillaries reabsorb the excessive tissue fluid and transport the fluid through the lymphatic pathway, and ultimately dispose it into the blood. When this extracellular fluid enters the lymphatic system it is called as lymph. It drains into larger vessels called the lymphatics. These vessels converge to form one of two large vessels called lymphatic trunks, which are connected to veins at the base of the neck. One of these trunks, the right lymphatic duct, drains the upper right portion of the body, returning lymph to the bloodstream via the right subclavian vein. The other trunk, the thoracic duct, drains the rest of the body into the left subclavian vein. The lymphatic pathway is an open circuit where lymphatic capillaries in body tissues reabsorb excessive tissue fluid which is derived from blood plasma.

Diagrammatic representation of Lymphatic circulation

Lymphatic circulation

This lymph ultimately returns to the blood plasma (i.e. blood plasma in capillaries → interstitial fluid → lymph in lymphatic pathway →lymph returns to blood plasma).

 

References:

  1. Immunology: Understanding The Immune System. By Klaus D. Elgert
  2. Immunology: A Short Course. By Richard Coico, Geoffrey Sunshine.
  3. Immunology.  By David K. Male.
  4. Immunology: Essential And Fundamental. By Sulabha Pathak, Urmi Palan.

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